1. Technical Field
The present invention relates to a power amplifier and, more particularly, to a circuit for preventing an element from being damaged although output impedance of a final transistor is changed in a power amplifier.
2. Description of the Related Art
In a power amplifier, damage to an output terminal element may be attributed to stability of a circuit, but, in most cases, it is ascribed to an electrical load due to a change in an output terminal end load.
Thus, a power amplifier requires a technique for protecting an output end element, and in a related art, a technique of sensing a current by using a replica circuit and protecting a circuit in the occurrence of an overcurrent in an output end according to the sensing result has been used. However, this technique disadvantageously has a low level of accuracy, a complicated circuit, and degraded efficiency.
Also, in another related art, in order to protect an output terminal element of the power amplifier, a voltage of an output terminal is sensed and a circuit is protected in the occurrence of an overvoltage in the output terminal according to the sensing result. However, in this technique, a change in impedance in the output terminal may degrade performance in a normal operation.
The present invention relates to a circuit for preventing damage to an element although output impedance of a final transistor in a power amplifier is changed, relates more particularly, to a circuit for controlling a bias voltage provided according to signal strength and an operational state by coupling a signal between a final output terminal of a transistor and a transformer, namely, a primary signal of the transformer.